Here's one for you:
All sizes of Line ships available all the time, instant buy-off time. You want a vanilla CA / CL / DD / FF etc., here it is, fly now.
All sizes of Command ships available one per turn, every turn.
Specialty ships on a variable basis, for example, no more than 1 F-CAD per day, 3 F-BCFs per day, etc. (admin selectable). The ship enters the shiplist a full day before it's actual release, and people bid on it via a "silent E-bay" system, ie, they punch in their max bid, and nobody sees the current max. If you made the actual highest bid of the day, poof, you get the ship.
If a specialty ship is sold back, then it re-enters the bidding pool for X amount of time...